Abstract/Summary

This paper reports the first evidence for significant
overpressures in the Otway Basin, southern Australia,
where most previous studies have assumed near-hydrostatic
pore pressures. Overpressures are observed in the Upper
Cretaceous Shipwreck supersequence in several wells in
the Voluta Trough, such as Bridgewater Bay–1, Normanby–1
and Callister–1. One of these wells penetrated successions
of Pliocene-Recent marine clastic sediments nearly 700 m
thick that were deposited rapidly in submarine channels
and that were probably carved during the late-Miocene
to early-Pliocene. Wireline and drilling data suggest that
overpressures present in Upper Cretaceous shales and
sandstones in the Belfast Mudstone and Flaxman and
Waarre formations developed either due to disequilibrium
compaction—where there is no evidence of hydrocarbon
generation and thick Pliocene stratigraphy is present—
or due to fluid expansion where there is evidence of
hydrocarbon generation and the Pliocene stratigraphy
is thin to absent. The two key factors that may indicate
abnormal pore pressures in Upper Cretaceous sediments
in the central Otway Basin are the thickness of Pliocene
stratigraphy and whether or not hydrocarbons are actively
generating from source rocks.